Polyolefin polymers and polymer blends are known for their utility in a wide variety of applications. In particular, many polyolefin polymers, including copolymers of propylene with other alpha-olefins such as ethylene, are well suited for use in applications requiring good stretchability, elasticity, and strength. Such polymers often comprise a blend of two or more propylene copolymers, and may be manufactured by physically blending two or more copolymers, or by reactor blending of the copolymers, or by reactor synthesizing two or more copolymers with a single catalyst having multiple reactive sits or with multiple catalysts.
Many polyolefin blends known in the prior art are formed into pellets for intermediate storage purposes before being shaped into articles such as fibers, films, nonwovens, extruded coatings, and molded articles. Some of these compositions, however, are known to exhibit poor pellet stability over extended periods of time, leading to agglomeration of pellets and resulting in pellet batches that a do not flow/pour well, particularly after storage and shipping under hot climate conditions. Further, the typically low melting points of such known polymer blends often lead to flattening or other deformation of polymer pellets during long-term storage, which also negatively affects the ability of the polymer pellets to be free-flowing. While blending such polyolefin copolymers with higher-crystallinity components has been shown to improve stability properties of the polymer pellets, such pellets lose some of their elasticity and still have a tendency to agglomerate during shipping and long-term storage, thus presenting processing issues where free-flowing pellets are required.
As a result, many known polyolefin blend pellets are blended with approximately 10 wt % of a crystalline random propylene-ethylene copolymer, as disclosed in U.S. Pat. Nos. 7,026,405 and 7,803,876. While the resultant polyolefin are pellet stable, they are generally less elastic with a higher tension set, top load, and hysteresis as compared to the original polyolefin blend pellets without the random copolymer. Accordingly, such products have limited utility in applications where enhanced elasticity is required.
The inventors have discovered that incorporating a low reactivity ratio product/alternating sequenced pyridyl diamido-based catalyzed copolymer with high reactivity ratio product/blocky sequenced metallocene catalyzed copolymer can produce a balance of a pellet stable bimodal propylene-alpha olefin copolymer having suitable elastic recovery properties. In contrast to products prepared with a random propylene-ethylene copolymer, the olefin copolymers of the invention have a broad split in molecular weight of each component but a narrow split in alpha-olefin content of each component.